where 0 is the slot number. I guess luks stores slots as 0,1,2 etc. But I recommend you to be very careful with luks as you may lock yourself out. Please refer to manual of cryptsetup. Also I recommend you to visit other distro IRC channel just to be extra safe before you proceed.

You may need to reboot the machine and try your new passsphrase as well before you remove the key.

cryptsetup luksRemoveKey <partition> actually takes a keyfile as the last parameter. So if you want to input the passphrase, just give the device as parameter. cryptsetup will first prompt for "Enter LUKS passphrase to be deleted:" and then "Enter any remaining LUKS passphrase:"
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user1338062Apr 17 '13 at 9:30